Coated metal pipe

ABSTRACT

A coated metal pipe which comprises a pipe coated with a composition comprising 5-30 wt parts of ethylene-vinylacetate copolymer having a vinylacetate content of 15-40 wt% and a melt index of 1-1,000 g/10 min; 2-20 wt parts of low molecular weight oxidized polymer of polypropylene or propylene-ethylene copolymer; 5-40 wt parts of styrene butadiene copolymer; 5-50 wt parts of a tackifier and 10-80 wt parts of asphalt.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal pipe coated with a high thermalstability layer which is characterized by excellent adhesiveness andrecoatability.

2. Description of the Prior Art

Heretofore, it has been proposed to coat a metal pipe to be laidundergound with an asphalt composition in order to make itanticorrosive. However, conventional asphalt compositions possessdisadvantages in that they can easily be peeled from the surface of themetal pipe or they are subject to deterioration which decreases theiranticorrosiveness. Accordingly, in order to avoid serious damage, ametal pipe possessing excellent anticorrosiveness, safety and durabilityis desired. This is especially true of metal pipe containing electricalcable such as telephone cable or metal pipe used for feeding gas.

It has been proposed to coat the metal pipe with a compositioncomprising asphalt, ethylene-vinylacetate copolymer, styrene-butadienerubber and rosin or petroleum resin in order to improve the coatingproperties. However, in this case, the anticorrosiveness, stability anddurability of the coated composition also deteriorate with age.

Additionally, it has been proposed to use various other coatingcompositions besides the asphalt composition. However, after the pipe islaid underground, cracks often form in these coated layers as a resultof vibration, thereby permitting serious corrosion in the underlyingpipe. Accordingly, it would be desirable to have a coated metal pipewhich has excellent capability for being recoated. When a coated metalpipe is laid underground, it is exposed to changing temperatures.Accordingly, the coated layer should be effective independent of thechange in temperature.

A need exists therefore for a coated metal pipe having theaforementioned desirable characteristics.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide acoated metal pipe for laying underground which is coated with a layerhaving excellent thermal stability, adhesiveness and recoatability.

This and other objects of this invention as will hereinafter be made byclear by the discussion below have been attained by providing a coatedmetal pipe for laying underground wherein the coating can be applied bya heat-melt process, wherein the coating comprises a composition of 5-30parts by weight of ethylene-vinylacetate copolymer having a vinylacetatecontent of 15-40 wt% and melt index of 1-1000 g/10 min; 2-20 wt parts byweight of low molecular weight oxidized polymer of propylene-ethylenecopolymer, ethylene vinylacetate copolymer or polypropylene; 5-40 partsby weight of styrene-butadiene copolymer; 5-50 parts by weight of atackifier and 10-80 parts by weight of asphalt. This composition will bereferred to as an oxidized polymer coating composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan sectional view of one embodiment of coated metal pipeaccording to the invention;

FIG. 2 is a plan sectional view of another embodiment of coated metalpipe;

FIG. 3 is a side sectional view of the coated metal pipe of FIG. 1; and

FIG. 4 is a side sectional view of the coated metal pipe of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, 1 represents the body of the coated metalpipe for laying underground. It can be a steel pipe, a cast iron pipe, awelded pipe, or the like. One end of the metal pipe 1 can be connectedto other pipes by means of a flange or a screw thread. The diameter andthe thickness of the metal pipe can be selected depending upon theparticular application.

Typical metal pipes for laying underground include the following: gasfeed pipes; metal pipes containing a cable, e.g., telephone line,telegraph line, electric feed wire, etc.; petroleum transportationpipes, water supply pipes; and other metal pipes. These metal pipes areusually made of steel or iron. Such pipes can be laid in cement,concrete or underground and, if necessary, can have a suitableprotective coating.

The oxidized polymer coating composition 2 is coated on the surface ofthe metal pipe 1. The thickness of the composition 2 is usually 0.1-10mm, preferably 0.1-1 mm and especially 0.2-0.5 mm. It is applied byheat-melting at 100°-250°C with subsequent coating onto the metal pipe.

One of the main components of the oxidized polymer coating composition 2is asphalt, such as straight asphalt, blown asphalt, bitumen, coal taror a mixture thereof. The asphalt comprises 10-80 wt%, preferably 20-50wt% of the total composition. If the content of the asphalt is less than10 wt%, the initial adhesive force of the composition will be inferior.On the other hand, when the content of the asphalt is higher than 80wt%, the cohesive force and thermal stability of the composition will beinferior.

Another principle component of the composition is theethylene-vinylacetate copolymer having a vinylacetate content of 15-40wt% and a melt index of 1-1000 g/10 min. The preferred copolymer has avinylacetate content of 20-30 wt% and a melt index of 50-500 g/10 min,preferably 100-500 g/10 min. The ethylene-vinylacetate copolymer ispresent in the coating in the range of 5-30 wt%, preferably 8-20 wt% ofthe total components.

The third main component of the composition is a low molecular weightoxidized polymer of propylene-ethylene copolymer, ethylene-vinylacetatecopolymer or polypropylene. It is preferred that the propylene-ethylenecopolymer contain an ethylene component of 3-15 wt%, and have an averagemolecular weight of 400-5000, preferably 1000-5000, as measured by thevapor osmotic pressure method. Its acid value should be 1-50, preferably2-7 as measured by the dioxane-tetralin solvent method (acid-alkalititration). It is preferable to use a low molecular weight oxidizedethylene-vinylacetate copolymer containing a vinylacetate component of5-40 wt% of the total composition and having an average molecular weightof 400-5000 as measured by the vapor osmotic pressure method. It is alsopreferable to use a low molecular weight oxidized polypropylene havingan average molecular weight of 400-8000, as measured by the vaporosmotic pressure method. The low molecular weight oxidized polymershould constitute 2-20 wt% of the total composition. When its content isless than 2 wt%, the softening point of the composition is too low. Onthe other hand, when it is present at higher than 20 wt%, themiscibility is inferior.

The low molecular weight oxidized polymer can be prepared by oxidizingpropylene-ethylene copolymer, ethylene-vinylacetate copolymer orpolypropylene with oxygen, air, an oxygen containing gas, ozone or aperoxide, e.g., potassium permanganate. Oxidation by an oxidizing gas,especially ozone, is economical. Thermal cracking and oxidation ofpropylene-ethylene copolymer, ethylene-vinylacetate copolymer orpolypropylene can be accomplished using ozone without any difficulty.When the oxidation of a low molecular weight propylene-ethylenecopolymer is carried out by using oxygen, an oxygen containing gas, orozone, etc., a colorless low molecular weight oxidizedpropylene-ethylene copolymer can be obtained by combining steam with thegas at 50°-300°C. A low molecular weight propylene-ethylene copolymercan also be prepared by telomerizing an olefin or by thermal cracking ofpolypropylene-ethylene copolymer. The thermal cracking method iseconomical, and can be performed by thermal cracking high molecularweight propylene-ethylene copolymer at higher than 300°C in an inertgas, e.g., nitrogen, carbon dioxide, argon, etc. or in saturated orsupersaturated steam.

The fourth principle component of the composition used in the inventionis a tackifier. Suitable tackifiers include natural rosin, rosinpolymers, rosin polyol esters, hydrogenated rosin polyol esters ormixtures thereof. The petroleum resins include aromatic petroleumresins, aliphatic petroleum resins, alicyclic petroleum resins, etc. (MW200-15,000, preferably 400-8000 as measured by liquid chromatography).They can be combined with the rosin. The terpene resins includeα-pinene, β-pinene or a mixture thereof or a mixture of β-pinene anddipentene (softening point (Ball & Ring method) 5°-230°C; MW 200-20,000,preferably 500-10,000). From the viewpoint of thermal stability, it isdesirable to use the aromatic petroleum resin and the rosins having asoftening point of higher than 70°C in order to keep high temperaturecreep to a minimum. The tackifier should be present at 5-50 wt%,preferably 10-40 wt% of the total composition. When the tackifier ispresent at less than 5 wt%, the adhesiveness and thermal resistance ofthe composition are inferior. On the other hand, when the tackifiercontent is more than 50 wt %, the adhesiveness of the composition isinferior and it becomes brittle.

The fifth main component of the composition used in the invention isstyrene-butadiene copolymer, which is preferably prepared by solutionpolymerization. The styrene butadiene copolymer, usually elasticstyrene-butadiene copolymer, is present at 5-30 wt%, preferably 8-18wt%, of the total composition. When the styrene-butadiene copolymer ispresent at less than 5 wt%, the recoatability of the composition isinferior. On the other hand, when the styrene-butadiene content is morethan 30 wt %, the processability is inferior. The recoatability of thecomposition is highly affected by the synergistic effect of the lowmolecular weight oxidized propylene-ethylene copolymer and thestyrene-butadiene copolymer.

In the preparation of the coated metal pipe, the above mentionedcomponents are mixed, melted and coated on the surface of the metalsurface. The five components can be mixed simultaneously for melting.However, it is preferable to heat-melt separately the high meltviscosity components, ethylene-vinylacetate copolymer and thestyrene-butadiene copolymer, by use of a kneader, such as a roll mill.The other components such as the asphalt, the tackifier and the lowmolecular weight oxidized propylene-ethylene copolymer are also meltedand blended. The former mixture is then blended with the latter mixturein the molten state to form the coating composition. It is possible tomodify the order of the mixing of the components. The temperature formelt-blending can be relatively low when the blending is carried out bya mechanical force mixing method with high shear strength. It ispreferred to heat the blend to a temperature slightly higher than thatat which all components are melted, such as 100°-250°C, preferably130°-170°C.

The coating of the metal pipe can be accomplished by various mechanicalmethods. The preferred technique is to coat the composition by heat-meltcoating without a solvent. A typical healt-melt coating method is theextrusion coater method in which the composition is charged into anextruder and is coated on to the surface of the metal pipe by moving themetal pipe in a die mounted at the end of the extruder. With thatmethod, the composition can be coated at the same time that it isprepared. Alternatively, it is possible to prepare blocks or pellets ofthe composition by heat-melt blending and then feed the blocks orpellets to the coater.

The process of coating the metal pipe usually involves the followingsteps: dust removal (steel or iron pipe surface treatment); preheating;pipe insertion (to coater); coating; and cooling. The coating of thecomposition is carried out at 100°-250°C. The thickness of the coatedlayer is usually 0.1-10 mm preferably 0.1-1 mm, especailly 0.2-0.5 mm.

In order to impart electrical insulation, chemical resistance, waterresistance, cold temperature resistance and recoatability to the coatedmetal pipe, it is possible to place an outer coating of a polyolefinthermoplastic resin such as polyethylene, polypropylene, vinylchlorideresin, etc. on the surface of the coated layer. It is especiallypreferable to coat high density or middle density polyethylene having adensity of 0.94-0.98 g/cm³ as the outer coating. Polyethylene having adensity of 0.91-0.94 g/cm³ can be used for coated metal pipes havingsmall diameters. The outer coating of the thermoplastic resin can beapplied by an extrusion method, a powdering method, or a tape bindingmethod, etc. The extrusion method can be used to apply the thermoplasticresin coating to small diameter metal pipes. As shown in FIG. 2 and FIG.4, the thermoplastic resin upper coating 3 is formed over the oxidizedpolypropylene copolymer coating composition 2. The thickness of theupper coat is usually 0.1-10 mm, preferably 0.1-2 mm, especially 0.3-1.0mm.

The coated metal pipe of the present invention will be illustrated belowwith reference to the following Examples which are provided herein forpurposes of illustration only.

EXAMPLES

The melt-viscosity, the R & B softening point, the adhesiveness, therecoatability recovering property, the fluidity and the thermalstability of the example coating compositions shown in the followingtables were measured by the following test methods.

Test Methods

1. Melt viscosity -- B type viscometer (150° C)[C.P.S.]

2. R & B softening point -- JIS K 2531 [°C]

3. Adhesiveness -- A coating composition which was heat-melted at 150°Cwas spread onto an iron plate preheated to 150°C. A spacer having athickness of 0.2 mm was placed over that and a sheet made ofpolyethylene was used to cover the coated composition. The test assemblywas cooled under a pressure of 10 kg/cm² and was left undisturbed for 24hours. The peel strength was measured from 20°-180°C with a peelingspeed of 50 mm/min. [kg/10 mm]

4. Recoatability -- A coating composition was heat-melted and was spreadonto an aluminum plate having a thickness of 0.5 mm. A polyethylenesheet having a thickness of 0.8 mm was placed over the coatedcomposition under finger pressure to form a coated composition having athickness of 0.3 mm. A hole having a diameter of 3 mm was formed on thepolyethylene sheet. The coated composition in the hole was taken outwith tweezers. The sample was kept in an oven at 35°C with thepolyethylene sheet on top and a weight of 2 kg on it. After 24 hours,the condition of the hole was observed [%].

5. Fluidity Test -- A coating composition sheet of 6 cm × 4 cm × 0.5 cmwas placed on a slant plate at an angle of 70°C in an oven at 90°C.After 3 hours, the distance over which the composition had flowed wasmeasured. [mm]

6. Thermal Stability -- A coating composition was kept at 150°C for 96hours. The change in melt-viscosity and the amount of the film formationwere observed.

In Table 1 and Table 2, the following terms are used.

EVA: ethylene-vinylacetate copolymer (vinylacetate content 25 wt%; meltindex 100g/10min)

SBR: styrene-butadiene copolymer (24:75) prepared by the solutionpolymerization method.

Rosin: Purified rosin glycerine ester

Oxidized PP: oxidized ethylene-propylene copolymer having an averagemolecular weight of 3,000 (Acid value 5)

Low MW PP: Polypropylene having an average molecular weight of 2,500

Low MW PE: Polyethylene having an average molecular weight of 2,500.

                  TABLE 1                                                         ______________________________________                                        Composition Example 1  Example 2  Example 3                                               wt%        wt%        wt%                                         ______________________________________                                        Asphalt     42         40         44                                          EVA         18         18          9                                          SBR         10         18         21                                          Rosin       23         20         20                                          Oxidized PP  7          4          6                                          ______________________________________                                        Measured                                                                      Properties                                                                    ______________________________________                                        Melt viscosity                                                                            9.000      25,000     40,000                                      [C.P.S.]                                                                      R & B softening                                                                           90         105        100                                         point [°C]                                                             Adhesiveness                                                                              10         12          8                                           [kg/10 mm]                                                                   Recoatability                                                                             100        100        100                                          [%]                                                                          Fluidity     4          2          2                                           [mm]                                                                         ______________________________________                                    

REFERENCE

The test results of the reference composition are as follows:

                                      REFERENCE TABLE 2                           __________________________________________________________________________           wt %                                                                   __________________________________________________________________________    Composition                                                                           1  2  3  4  5  6  7  8  9 10                                          Asphalt                                                                              70 22 50 50 18 50 22 85 42 42                                          EVA    30  7 35  5 35  2 18  2 18 18                                          SBR    -- --  5 35 35  2 15  3 10 10                                          Rosin  -- 36  5  5 10 42 20  5 23 23                                          Oxidized                                                                      PP     -- --  5  5  1  2 25  5 -- --                                          Isoprene                                                                             -- 29 -- -- -- -- -- -- -- --                                          Low MW PP                                                                            -- -- -- -- -- -- -- --  7 --                                          Low MW PE                                                                            -- -- -- -- -- -- -- -- -- 7                                           __________________________________________________________________________    Properties                                                                    __________________________________________________________________________    Fluidity                                                                             -- -- -- -- -- too                                                                              -- too                                                                              too                                                                              too                                                               high  high                                                                             high                                                                             high                                        Adhesive-                                                                            bad                                                                              -- -- -- -- bad                                                                              bad                                                                              bad                                                                              bad                                                                              bad                                         ness                                                                          Recoata-                                                                             bad                                                                              -- bad                                                                              -- bad                                                                              -- bad                                                                              -- bad                                                                              bad                                         bility                                                                        Thermal                                                                              -- bad                                                                              -- -- -- bad                                                                              -- bad                                                                              -- --                                          stability                                                                     Process-                                                                             -- bad                                                                              -- bad                                                                              bad                                                                              -- -- -- -- --                                          ability                                                                       Miscibility                                                                          -- -- -- bad                                                                              -- -- -- -- bad                                                                              --                                          Melt-  -- -- -- -- too                                                                              -- -- -- -- --                                          viscosity          high                                                       __________________________________________________________________________

The above results correlated well with the test results observed whenthe composition was coated on a steel pipe.

The coated metal pipe of the invention has a coated layer possessingexcellent recoatability, adhesiveness, heat resistance and coldresistance. When the coating is cracked, the coated layer around thecrack flows naturally to recover the crack. The anticorrosiveness isalso excellent, as is the safety of the coated metal pipe when placedunderground. Additionally, the coated layer resins stability independentof temperature changes likely to retains during underground usage. Thisis a great advantage in the case of gas pipes for example, since itmakes gas leakage unlikely.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spriit or scope of the inventionas set forth herein.

What is claimed as new and intended to be covered by Letters Patentis:
 1. A coated metal pipe which comprises a pipe coated with acomposition comprising 5-30 wt parts of ethylene-vinylacetate copolymerhaving a vinylacetate content of 15-40 wt% and a melt index of 1-1000g/10 min; 2-20 wt parts of low molecular weight oxidized polymer ofethylene-vinylacetate copolymer, polypropylene or propylene-ethylenecopolymer; 5-30 wt parts of styrene-butadiene copolymer; 5-50 wt partsof a rosin tackifier and 10-80 wt parts of asphalt.
 2. The coated metalpipe of claim 1, wherein said low molecular weight oxidized polymer is alow molecular weight oxidized propylene-ethylene copolymer.
 3. Thecoated metal pipe of claim 1, wherein said low molecular weight oxidizedpolymer is a low molecular weight oxidized ethylene-vinylacetatecopolymer.
 4. The coated metal pipe of claim 1, wherein said lowmolecular weight oxidized polymer is a low molecular weight oxidizedpolypropylene.
 5. The coated metal pipe of claim 1, wherein thecomposition is coated onto the surface of a pipe, the coating having athickness of 0.1-5 mm.
 6. The coated metal pipe of claim 1, wherein themetal pipe coated with the composition is overcoated with a polyolefinthermoplastic resin having a thickness of 0.1-10 mm.
 7. The coated metalpipe of claim 1, wherein said low molecular weight oxidized polymer hasa molecular weight of 400-5000 and an acid value of 1-20 measured by thedioxane tetralin solvent method.
 8. The coated metal pipe of claim 1,wherein the composition is heat-melt coated onto the metal pipe body at100°-250°C to form a coated layer having a thickness of 0.1-10 mm. 9.The coated metal pipe of claim 1, wherein the tackifier is rosin, rosinpolymer, rosin polyol ester, hydrogenated rosin, hydrogenated rosinpolyol ester, petroleum resin or terpene resin.
 10. The coated pipe ofclaim 1, wherein the coated layer is overcoated with a polyolefinthermoplastic resin.
 11. A coating for metal pipe which comprises 5-30wt parts of ethylene-vinylacetate copolymer having a vinylacetatecontent of 15-40 wt% and a melt index of 1-1000 g/10min; 2-20 wt% partsof low molecular weight oxidized polymer of ethylene-vinylacetatecopolymer, polypropylene or propylene-ethylene copolymer; 5-30 wt partsof styrene-butadiene copolymer; 5-50 wt parts of a rosin tackifier and10-80 wt parts of asphalt.
 12. The coating of claim 11, wherein said lowmolecular weight oxidized polymer is a low molecular weight oxidizedpropylene-ethylene copolymer.
 13. The coating of claim 11, wherein saidlow molecular weight oxidized polymer is a low molecular weight oxidizedethylene-vinylacetate copolymer.
 14. The coating of claim 11, whereinsaid low molecular weight oxidized polymer is a low molecular weightoxidized polypropylene.
 15. The coating of claim 11, wherein said lowmolecular weight oxidized polymer has a molecular weight of 400-5000 andan acid value of 1-20, measured by the dioxane tetralin solvent method.16. The coating of claim 11, wherein the tackifier is rosin, rosinpolymer, rosin polyol ester, hydrogenated rosin, hydrogenated rosinpolyol ester, petroleum resin or terpene resin.